1. Field of the Invention
The present invention relates to a fixing apparatus for fixing a toner image on a sheet and an image forming apparatus provided with this fixing apparatus.
2. Description of the Related Art
In an image forming apparatus, generally, an unfixed image formed on a sheet by toner is heated and pressed in a fixing device, and is fixed on the sheet surface. The fixing device is heated by an inside heater, and the heater is controlled to maintain a temperature necessary for fixation while supplementing heat quantity absorbed by a sheet that passes through there.
Kinds of a sheet material transported by an image forming apparatus have been increased year by year, and in a structure in which image fixation is performed in one fixing device, it has become difficult to make a stable fixing characteristic, good quality of a fixed image, and their productivity consistent with each other for all of these materials. In order to cope with this circumstance, a technique has been employed in which a plurality of fixing devices are arranged in series on a transport path to avoid a problem caused by the structure of one fixing device, such as lack of heat quantity (Japanese Patent Application Laid-Open No. 6-348159 and Japanese Patent Application Laid-Open No. 7-271226).
Meanwhile, there are materials, such as paper called a plain paper whose basis weight is small and a second surface of a thick paper (where the water content has decreased and the sheet temperature has increased), which fixing can be sufficiently performed with only one fixing device, and such a material of sheet could wind around a curl or a fixing roller because an excess heat quantity is applied thereto when it passes through two or more fixing devices. In order to cope with a problem about such a material, it has been considered to provide a bypass transport path that bypasses a fixing device on a downstream side among a plurality of fixing devices and thus branches from a main transport path that passes through two fixing devices. In the way the material, which causes the problem when it passes through two or more fixing devices, passes through the bypass transport path (U.S. Pat. No. 6,512,914, Japanese Patent Application Laid-Open No. 2001-005319, Japanese Patent Application Laid-Open No. 5-158364, and Japanese Patent Application Laid-Open No. 7-191564).
However, in a case where the bypass transport path is heated by convection heat coming from a fixing device that the bypass transport path bypasses, a problem as described below occurs. Since heating of the fixing device is carried on regardless of whether the image forming apparatus is in a standby state or in an image forming state, the bypass transport path is constantly heated by the fixing device that the bypass transport path bypasses while the image forming apparatus is turned on.
In the case, when the bypass transport path is heated, the bypass transport path comes in contact with a sheet passing through the bypass transport path, and toner which has been fixed already on a sheet melts again, resulting in image deterioration. Besides, the melted toner sticks to the bypass transport path, and due to this sticking toner, a jam occurs. Further, the melted toner adheres to a transport roller provided in the bypass transport path and causes contamination on the transport path, so that a transport defect occurs or frequent service maintenance is required.